Certain low level spiders in the Steamfont Mountains don't appear to have pathing. The spiders I have noticed this on are to the left of Watchmen Dreeb (assuming you are heading out away from Ak'anon) after you follow the path to it's end go left into the canyon. Here is a screen shot of the spiderlings that don't move.
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Another thing I noticed with these spiderlings is that when you kill them they always respawn the same level that they were when you killed them. For example the one on the right in the picture is always level three while the other three are always level two.